1. Field of the Invention
The present invention relates to an outboard motor and a watercraft including the same.
2. Description of the Related Art
Conventionally, water of the sea, river or the like (hereinafter, referred to as “external water”) on which a watercraft is traveling is used to cool an engine of an outboard motor of the watercraft.
An engine includes a cylinder block having a cylinder in which a piston is slidably accommodated and also includes a cylinder head attached to a tip portion of the cylinder block so as to cover the cylinder. The cylinder, the piston and the cylinder head form a combustion chamber. The cylinder head has an intake port and an exhaust port facing the combustion chamber. The exhaust port is connected to an exhaust pipe for discharging exhaust gas. In the cylinder block, the cylinder head and the exhaust pipe, a water jacket for causing external water acting as cooling water to flow is formed.
External water does not exist for the purpose of cooling engines. The temperature of the external water is lower than the temperature of cooling water for cooling an engine of an automobile or the like. Usually, the temperature difference between the engine and the external water is significantly large. Therefore, if the external water is caused to flow in the water jacket of the cylinder block, the water jacket of the cylinder head and the water jacket of the exhaust pipe in this order, the cylinder block may be cooled excessively at the time of start of the engine, which may deteriorate the warm-up characteristics. When the temperature of the cylinder block is too low, the viscosity of a lubricant provided between an inner wall of the cylinder and the piston is likely to be raised, which increases the resistance against the reciprocating motion of the piston. This may undesirably increase the engine loss and thus decrease the fuel efficiency. By contrast, excessive cooling of the exhaust pipe does not specifically cause any adverse influence. Thus, it has been proposed to cause the external water to flow in the water jacket of the exhaust pipe, the water jacket of the cylinder head and the water jacket of the cylinder block in this order (see Japanese Patent No. 2791876).
However, in the outboard motor, after the warm-up is finished, the temperature of the exhaust pipe is likely to be raised in addition to the temperature of the cylinder head and the temperature of the cylinder block. With the above-described conventional technology, the temperature of the external water becomes significantly high when flowing in the water jacket of the exhaust pipe. As a result, the temperature of the external water supplied to the water jacket of the cylinder head is significantly high. This causes a problem of deteriorating the cooling performance of the cylinder head and the cylinder block.
Aside from the above technology, it is conceived to cause all of the external water to flow in the water jacket of the cylinder head, the water jacket of the cylinder block and the water jacket of the exhaust pipe in this order. In this case, the water jacket of the exhaust pipe is supplied with the high-temperature external water which has cooled the cylinder head and the cylinder block. However, as described above, in the outboard motor, the temperature of the exhaust pipe is likely to be raised. This may raise the temperature of the water supplied to the water jacket of the exhaust pipe and thus the exhaust pipe may not be cooled sufficiently. As a result, the temperature of the outboard motor may be undesirably high.